Intersected Station

From TrigpointingUK
Jump to: navigation, search

Intersected Stations

spire
chimney
vane
pinnacle

An intersected station is one at which the coordinates were derived by intersection, no observations were ever taken at the station. Intersection is a process where rays are observed to the station from many outside points and the coordinates derived from the resulting intersection of those rays

The main distinguishing factor is that intersected stations generally cannot be occupied by a theodolite.

However there is the noticeable exception of St Pauls Cathedral. This was originally intersected during the Principal triangulation and later (during the Principal) a scaffold was erected and the Ramsden 36" theodolite taken to the top and observations taken. [1].

Examples of Intersected stations include church spires, weather vanes, pinnacles and chimneys. They were often used in conjunction with a nearby mark that could be 'occupied', for example a bolt on the church roof.

The following table is extracted from [2] using records where the station name contains "I" (denoting that it comprises only 'Intersected' marks). In many other cases, a station contains both Intersected and Occupiable marks; here, the station name does not contain "I" and the data is not included in this table.

The inclusion of a block as an intersected station may seem surprising. The single instance is Lochboisdale Pier for which the OSGB36 record states "Centre of conc block on pier". This suggests that it was not an OS block with a metal mark but a pre-existing non-OS concrete block with a mark type of 'Centre'.

The three instances of 'Stone', Chars Stone, East Breakwater North End and West Breakwater East End are all recorded as destroyed but may have been something similar.

It is unclear why the rivet at John Lea Sec Mod Sch Wtr Twr (SK22I035) or the bolts at Park Place Belfry (SU77I011) and Laurence House (NT48I103) were designated as an Intersected Station.

Summary of Ordnance Survey intersection stations by type
type count
apex 35
spire 2523
tower 67
flagstaff 268
mast 280
chimney 741
belfry 93
vane 441
beacon 41
lighthouse 182
dome 101
cross 37
memorial 15
obelisk 18
cupola 107
pinnacle 31
monument 107
centre 68
pipe 3
cairn 13
old trig 20
pylon 15
l'ning c 11
pole 6
stone 3
vent 11
aerial 6
rivet 1
bolt 2
block 1
turret 4
lamp 4
total 5255


References

  1. Email from Ian Wilson to the Trigonomy mailing list, 4 July 2012
  2. https://www.ordnancesurvey.co.uk/legacy/docs/gps/CompleteTrigArchive.zip